1. Field of the Invention
The invention relates to information storage systems, and more particularly, to magnetic information storage systems.
2. Description of Related Art
Conventional magnetic storage devices are generally of the continuous magnetization type, wherein units of information, representing information bits, take on the form of successive discrete magnetic fields each having an orientation which is either in alignment or in opposition to the next field in the succession. A read head passing over these fields detects a voltage peak when encountering a pair of successive bits having opposite orientations due to the repulsive forces between the fields. By contrast, absence of a peak at an expected boundary between fields indicates identical orientation of the successive fields. More specifically when successive bits both have either a North-South orientation on the one hand, or a South-North orientation on the other, then no voltage peak is sensed by the reading head, indicating a first logic state. Conversely, when successive bits respectively have South-North followed by North-South orientations on the one hand, or North-South followed by South-North orientations on the other, the read head detects a voltage peak and registers this as a second logic state. In this manner, two bit states can be identified and a binary information storage system implemented.
Directionally, the alignment of the bits in conventional continuous magnetization storage devices is in parallelism with the bitstream direction. That is, the magnetic fields of the bits are aligned in the direction of relative travel between the reading (or writing) head and the storage medium, with each field being either in the North-South direction or in the South-North direction. In this sense, information storage in conventional devices is one-dimensional, with the single dimension extending in the bitstream direction.
The one-dimensional limitation of the prior art is in part imposed by the continuous nature of the storage medium. By disposing the magnetic fields contiguously with each other, interactions between the fields preclude practical implementations of field alignment in directions other than along the bitstream direction. Hence, only two bit states, as described above, are possible.
In any magnetic recording medium, there must be some minimal spacing between adjacent magnetic domains or else it will be impossible to separately write and read the magnetic state of an individual magnetic domain because of interference from adjacent, abutting, or overlapping domains. Crosstalk and mutual magnetization occurs in which information is lost, rendering a magnetized domain useless. The magnetized domain becomes machine unreadable. Ordered data in the form of a series of sequential, magnetized holes is impossible.
U.S. Pat. No. 4,393,110 discloses that holes are formed in a non-magnetic substrate by electron beams or by laser beams. The holes are subsequently filled with magnetic material. The holes have diameters between 200 A and 5000 A. Holes that are larger or smaller than this range of diameters are not suitable for the disclosed magnetic recording medium. There are between 1.6xc3x97107 and 1.6xc3x971011 holes/cm2. Greater or lesser densities are not suitable for the disclosed magnetic recording medium. The magnetic recording medium has a (linear) recording density of 100 KBPI (kilobits per inch).
Electron beam systems involve the movement of an electron gun or the movement of a mechanical stage. Variations that arise in the fabrication of holes using an electron beam system include: 1) variation of the stage movement and placement of the substrate and system mechanical components; 2) variations in the nature of the beam; and 3) variations in the process of hole creation in the sense of limits imposed by physics and chemistry.
Known reading/writing heads for magnetic recording media are only capable of reading/writing information when the information is in specific locations on the magnetic recording medium. In a typical reading/writing arrangement, a head is held in position over a rotating magnetic recording medium and magnetizes or detects the magnetic state of particular magnetic domains (i.e., areas of magnetized material corresponding to a single bit of information) of the magnetic medium moving relative to the head. The head is also often movable, such as by being movable radially outwardly and inwardly over the disk. Successive magnetic domains represent ordered data. If, when a disk is moved relative to a head, no magnetizable material, or insufficient magnetizable material, is disposed beneath the head, or if magnetizable material is not disposed sufficiently close to the head, the head cannot read or write on the disk. Information storage is impossible with the non-symmetrical domains because the distance of the head from the non-symmetrical domains permits only some of the domains to be written on so that information stored in the domains is machine readable. Unless all of the successive magnetic domains are capable of being written on and read, information storage is impossible.
Because of the particular physical arrangement of the Fukuda system, that system essentially discloses a magnetic recording medium of the type having a substantially continuous layer of magnetic material in the form of numerous holes filled with magnetic material, and wherein bits of information are stored by magnetizing successive, spaced groups of the filled holes. For this type of magnetic recording medium, typically, a head detects a magnetic state within an area on the surface of a disk medium because, within the area, there is likely to be a group of the small, filled holes that can be magnetized and detected. Given the limits of electron beam technology and the disclosure of Fukuda, Fukuda does not disclose a magnetic recording medium where each hole filled with magnetic material corresponds to one bit of information because such a magnetic recording medium would have overlapping or interfering magnetic domains.
In disk media having numerous randomly located holes filled with magnetic material, the holes are sufficiently numerous and close together such that, although the filled holes are discontinuous, the overall effect of the holes is to simulate continuous magnetic material. A head vertically or horizontally magnetizes (i.e., writes) and/or detects the magnetic state of transitions (i.e., reads) between radially and/or concentric groups of the filled holes. In vertical recording media, a magnetic head device magnetizes successive, contiguous magnetic regions of recording medium at right angles to the surface of the medium, i.e., an axis between N-S poles of a magnetized region extend perpendicularly to the surface of the medium. In horizontal recording media, transitions in a magnetically recorded waveform follow each other on a tape or disk with N-S and S-N transitions recorded end to end. In both vertical and horizontal recording media, a transition in polarity between successive regions of the recording medium represents a bit of information. For example, a N-N or S-S transition might correspond to a xe2x80x9c1xe2x80x9d and a N-S transition might correspond to a xe2x80x9c0xe2x80x9d, or vice versa. Known writing and/or reading technology requires that at least one of the head and the disk be moved such that the head passes over the specific radial and concentric areas of the disk to read successive bits of previously written information.
Existing writing and/or reading technology is incapable of detecting the magnetic state of individual ones of randomly located filled holes, since there is no ordered succession in radial or concentric directions of the filled holes. No known reading and/or writing head is configured to identify the location of each successive filled hole or to magnetize or detect the magnetic state of the magnetizable material filled in the holes for such disks. Moreover, the individual filled holes are ordinarily so small and densely packed relative to one another that existing heads are too insensitive to magnetize or detect the magnetic state of individual ones of the holes.
In known disk media having continuous magnetic surfaces (often actually a layer of magnetic material beneath other layers closer to the actual surface of the disk) or magnetic surfaces defined by numerous randomly located holes filled with magnetic material, recording density is limited since adjacent continuous or effectively continuous magnetic regions interfere with each other. In horizontal recording media, so-called xe2x80x9cNeel spikesxe2x80x9d tend to form between adjacent north and south poles, thus reducing the energy stored. At high packing densities, the transitions occasionally touch one another and bridges may develop into separate islands.
It is preferred that the length of the particle-like magnets defining the information stored on conventional disk media be several times greater than their thickness to minimize self-demagnetization due to neutralization of adjacent poles of the magnets by one another. Particularly at high packing densities, there is an increased likelihood of such demagnetization.
Recording of data in disk drives is done by writing at a level that simultaneously erases old data. This erasure is nowhere near perfect, and a residual signal from old data is common. The ratio of the new signal to the old signal is called xe2x80x9coverwritexe2x80x9d. It is roughly proportional to the write level of the new signal. In conventional technology, edges of tracks are not perfectly overwritten due to spindle runout and insufficient write-wide-read-narrow margin, the new signal tends to be modulated by the old signal, and the new recorded track may have equal signal strength to the old track leading to bit error.
U.S. Pat. No. 4,893,299 appears to suggest etching a surface of a substrate 1 to provide a surface texture consisting of, e.g., two series of alternating grooves and ridges, the series preferably being at right angles to one another to form a xe2x80x9ccrossed gratingxe2x80x9d. Col. 6, lines 37-51. A thin film 2 of magneto-optic material is deposited on the surface. As shown in FIGS. 5a, 5c, 6a-6b, 6d-6e, 7a-7b, 7d, and 8, the film of magneto-optic material 11 may be discontinuous. The embodiments shown in FIGS. 6d, 7a-7b, and 7d show magneto-optic material 11 disposed only on crests of sinusoidal or tops of square-wave profile ridges, which may be parts of a crossed grating arrangement of ridges.
U.S. Pat. No. 4,985,885 appears to suggest an optical memory device A including a plurality of strip-shaped magneto-optic recording layers 2 which can be formed, by etching, Col. 6, lines 17-33, into concentric circles or spirals, or dots on virtual concentric circles or spirals. Col. 4, lines 13-18. Information is stored on the recording layers with laser light and stored information is read with laser light.
Another concern with magnetic storage media involves the physical interaction between the read/write head and the storage medium itself. Magnetic disk drive assemblies as used for mass data storage in computers and electronic systems today comprise either rigid (xe2x80x9chardxe2x80x9d) disk drives or flexible (xe2x80x9cfloppyxe2x80x9d) drives. Both types of drives incorporate low cost data storage capacity with rapid recovery of stored data. This rapid availability of stored data is a function of the rotational speed of the disk relative to the read/write transducer as well as the proximity of the transducer to the magnetic medium. In practice, a read/write transducer is mounted in a head assembly that accurately follows the surface of a disk at flying heights of less than 1 micron. In particular, the head suspension assemblies are designed to prevent contact between the read/write head and the magnetic recording medium during operation; such contact, called head crash, can destroy a read/write head and the magnetic medium in a short time due to the friction that results from the high rotational speed of the disk relative to the head. Although current technology provides lubrication and protective layers on the disk, these measures are generally intended to compensate only for transient friction events during stop/start cycles.
In general, control of the texture characteristics of the disk surface is required to reduce the substantial attractive forces that are generated between the read/write head and the stationary disk surface. Smoother disk surface textures result in higher attractive forces that prevent proper head liftoff and flying characteristics when disk rotation is commenced. Current disk manufacturing techniques must also assure that the disk surface roughness does not exceed certain upper-bound values; if excessive surface roughness results from the texturing process, undesirable increases in flying height also limit the density with which data can be stored on the disk. A central issue in current disk texturing processes is the reliability and consistency with which the desired surface roughness is obtained. The disk surface texture is typically characterized in terms of an arithmetic average roughness value (Ra). Current disk texturing processes generally produce Ra values in the range of 10-200 nm; the most modern disk drives achieve head flying heights of 0.2-0.3 microns with Ra values of approximately 10-50 nm. These texturing processes utilize special abrasives for producing circumferential patterns of scratches on the surface of metallic (predominantly aluminum) disk substrates which inevitably create surface feature extremes in the form of peaks and valleys. U.S. Pat. Nos. 4,996,622, 4,939,614 and 4,931,338 describe variations of this general process. Several of these patents propose different textures for separate areas of the disk optimized for stop/start operations and for read/write operations. These patents document the difficulty of obtaining low flying heights (i.e., less than 0.3 microns) while simultaneously achieving acceptably low head/disk attractive forces with current disk texturing processes.
Other texturing processes combine abrasive texturing processes with chemical processes. For example, U.S. Pat. No. 4,985,306 describes a recording disk produced by subjecting a base plate containing S1O2xe2x80x94Li2Oxe2x80x94Al2O3 series crystallized glass to crystallizing treatment, polishing the surface of the base plate to attain a surface roughness of 15 xc3x85 to 50 xc3x85 to evenly distribute, regularly and two-dimensionally, very fine and uniform crystal grains in the amorphous layer. The base plate is then etched with an etchant having different degrees of dissolution with respect to the crystal grains and the amorphous layer to form uniform and regular convexities and concavities on the surface of the base plate. A magnetic film and a protective layer are applied over the base plate. Because the system described in this patent relies on an abrasive texturing process for distributing crystal grains, there is an inevitable randomness to the ultimate distribution of concavities and convexities.
The trend toward smaller diameter disks has also presented difficulties for prior-art manufacturing techniques. It has become progressively more difficult to achieve the required consistency in Ra values and in disk flatness with decreasing disk diameter using conventional methods. Disk flatness variations cause axial runout of the read/write head during disk rotation. In current disk drives it is desirable to maintain this axial runout value at less than 1-2 microns. Conventional abrasive texturing techniques applied to current metallic disk substrates are becoming less viable as disk diameters are progressing downwards.
The invention overcomes deficiencies of the prior art by providing a magnetization format in which the orientations of the magnetic fields on the storage medium are not restricted to parallelism with the bitstream direction, but rather can extend in any direction in three-dimensional space. Using more than two possible machine readable orientations and assigning to each orientation a unique logic level achieves an information storage system whose numerical base is other than twoxe2x80x94that is, instead of a binary system, any base system can be realized so long as the read head is able to distinguish between the different orientations encountered.
A design of the read head in accordance with the invention comprises a pair of non-parallel, and preferably orthogonal heads which may be of the conventional inductive or magnetoresistive types. So configured, the heads, collectively referred to as a dual-axis read/write head and singularly referred to as uni-axis component read/write heads, are able to detect or write any orientation of a magnetic field lying on the storage medium in a plane parallel to that in which they lie. In effect, the magnetic fields, each disposed in a discrete domain separated from other such domains by a non-magnetizable separation material, operate as rotating magnets whose orientation is sensed by the dual-axis read/write head and correlated to a particular logic state of the storage system. The numerical base of the system is determined by the number of possible orientations of this xe2x80x9cmagnet.xe2x80x9d Moreover, an additional logic state can be provided by using a non-magnetized state of the domain.
One advantage of departure from the conventional binary format storage is increased storage density. Whereas in a binary system a byte of information, which comprises 8 bits, can store up to 256 (28) characters, in a base three system, for example, a byte can store 6,561 (38) characters, and in a base 10 system a byte can store 100,000,000 (108) characters.
To achieve the desired characteristics of the storage medium, such as an appropriate separation between the discrete domains, the materials of the medium are selected such that the domains, which are themselves magnetizable, are non-continuous and separated by a non-magnetizable separation material. Each domain is individually addressable and represents one bit of information. The domains, correlated physically on the storage medium as features, can take the form of discrete islands or of holes formed in the disk medium. Manufacture of such a storage medium, which is disclosed in co-pending U.S. patent application Ser. No. 08/159,552 now U.S. Pat. No. 5,768,075, to Bar-Gadda and assigned to the same assignee, and which is incorporated herein by reference in its entirety, may rely on etching or other semiconductor device manufacturing processes.
The discrete nature of each domain further enables an information storage and retrieval format in which the net magnetization of each domain is used to indicate a particular logic level. This represents a significant departure from conventional systems, which rely on magnetic state transitions between successive bits to code information, rather than coding the information based on the net magnetic state of each bit individually, irrespective of the state of other bits, as performed in the present invention.
Also in accordance with an embodiment of the invention, each domain is used to store a varying magnetic field having a particular waveform. Different waveforms, such as a square waves, sawtooth waves, etc., are each assigned a logic state to implement an information storage and retrieval system. DC and/or AC biasing are optionally imparted to the stored waveforms to improve signal-to-noise ratio. Additionally, the system can operate in the frequency domains, relying on for example LaPlace or other transforms to simplify information handling.
The present invention provides a disk medium that is formed, by the processes generally associated with the shaping of layers in semiconductor chip products, to have great capacity to store information. A pattern of designed, individually magnetizable features is formed on a substrate, the features corresponding to individual bits of information. The features are physically separated from one another by non-magnetic material, thereby minimizing certain problems associated with magnetic information storage systems. Particularly, the effects of crosstalk between adjacent magnetic bits are minimized, such as the need to maintain head flying heights as low as possible in order to magnetically read and write on a disk on which large quantities of information can be stored is reduced and the need to use high coercivity magnetic materials for forming the magnetizable features in order to store large amounts of information on the disk is reduced.
The present invention further provides a disk medium that, in one embodiment, contacts a magnetic head with a low coefficient of friction, provides smooth and stable travel performance in conjunction with a magnetic head for prolonged periods of time, has improved wear resistance in use and stability in storage environments, and is capable of consistent reproduction. In practice, a magnetic disk medium according to the present invention optimizes operational conditions in a system for reading information stored on the magnetic disk medium. Surface roughness characteristics of the magnetic disk medium are controlled by the processes generally associated with the shaping of layers in semiconductor chip products. These processes permit formation of disks having an average surface roughness that creates particular aerodynamic effects when the disk is rotated at particular speeds, the aerodynamic effects being useful for suspending a magnetic head at a desired flying height above the surface of the disk during read/write operations. The same processes permit control of surface characteristics relating to friction effects between the disk surface and a magnetic head during start-up and stopping of rotation of the disk. Information storage density characteristics of the magnetic disk medium may be raised to substantially whatever density is capable of being written on or read by a magnetic head of a magnetic disk assembly, the limits on the readable density being primarily those associated with conventional apparatus operational conditions, many of which, such as magnetic head flying height, are controllable through optimization of surface roughness characteristics of the magnetic disk medium of the present invention. Further, the magnetic disk medium is readable by fixed head assemblies.
The magnetic disk medium according to an embodiment of the present invention is textured without relying on known abrasive techniques. The texturing is controlled, and is therefore less susceptible to random variations of known texturing methods. There is, consequently, a consistently reproduced disk manufactured with the above method. Because the flying height of a magnetic head can be set at a known, lower height than in systems using disks manufactured by known methods, and because transient friction events can be minimized, the disk produced by a method according to the present invention is capable of storing information with a greater density than in known disks.
In accordance with one aspect of the present invention, a method for handling information is described. In the method, a designed topography is etched in a disk. Individually magnetizable features are formed on the disk, the features corresponding to the designed topography. Information is stored on the disk by selectively changing a magnetic state of individual ones of the features.
In accordance with another aspect of the present invention, a disk medium comprises an etched, designed topography in a disk, and individually magnetizable features formed on the disk and corresponding to the designed topography.
In accordance with another aspect of the present invention, a memory system includes a disk medium including an etched, designed topography in a disk and individually magnetizable features formed on the disk and corresponding to the designed topography. Means are provided for magnetizing individual ones of the features and means are provided for detecting the magnetic state of the features.
In accordance with yet another aspect of the present invention, a method for making a disk medium includes the steps of etching a designed topography in a disk and forming individually magnetizable features on the disk, the features corresponding to the designed topography.